Lift mast assemblies typically have a first pair of spaced apart uprights which are pivotally mounted to one end of a material handling vehicle and a second pair of spaced apart uprights which are mounted on the first pair of uprights and guided by the first pair of uprights for extensible movement relative thereto. Typically, such lift mast assemblies have a carriage which is mounted on the second pair of uprights and elevationally movable therealong for lifting a load. Lift mast assemblies of this type normally have a plurality of lift chains which transfer motion from a mast lift jack to the carriage and elevationally move the carriage relative to the movable uprights. Several problems are associated with the use of lift chains. One problem in particular relates to obstruction of operator visibility caused by the existence of the lift chains within a window established by the space between the second pair of uprights. This tends to clutter the area between the uprights and reduce the size of the window and operator's visibility.
Chains of the type used in lift mast assemblies are normally provided in pairs. These chains cooperate with a carriage lift jack or mast lift jack to elevationally move the carriage along the second pair of uprights and the mast along the first pair of uprights. The chains have a tendency to become unevenly adjusted in length as normal wear of the chain and cooperating component parts occur. This wear is usually a result of chain loading and the considerable number of operational cycles the chain must endure during normal operation. This uneven adjustment in the chains will result in cocking of the carriage and/or second pair of uprights relative to the uprights upon which it rides and cause undesirable side loading of the carriage, uprights, jacks and associated componentry. This will increase the potential for premature wear of the uprights (flaking galling and the like), premature wear of carriage and upright rollers and other loaded componentry, and reduced chain life. Also, improper chain adjustment during assembly or repair of the lift mast assembly can cause uneven length in the pairs of chains which will result in reduced life of the chains and associated carriage, upright and lift mast componentry indicated above.
Cocking of the carriage, due to uneven chain length, will also cause greater frictional forces to occur at the carriage rollers. This will increase the amount of horsepower required to elevate the carriage and reduce the efficiency of operation of the lift system. Excessive carriage cocking will make it difficult for the operator to align the forks mounted on the carriage with the load to be lifted. During lifting of the load, the cocking of the carriage will cause the load to be at an angle of inclination in a direction traverse the pair of uprights. The added weight of the load on the carriage will serve to increase carriage side loading and premature wear and failure of the associated componentry.
In order to achieve full free lift, that is elevational movement of the carriage from the fully lowered position adjacent a lower end of the second pair of uprights (movable) to a fully raised position adjacent the upper end of the second pair of uprights, an additional lift jack (a carriage lift jack) which is mounted on the on the second pair of uprights is required. This carriage lift jack cooperates with the lift chains to cause the lift chains to move at a ratio of two to one and move the carriage along the second pair of uprights without requiring any of the extension of the second pair of uprights relative to the first pair of uprights. This is important especially in areas wherein overhead clearances are low and elevation of the carriage assembly without elevation of the movable uprights is a necessity. This, of course, clutters up the lift mast assembly between the spaced apart second pair of uprights (window) and further reduces visibility of the vehicle operator.
One attempt to solve the above noted problems is disclosed in U.S. Pat. No. 4,356,893 dated Nov. 2, 1982 to Richard J. Johannson. This patent discloses a lift mast assembly in which lift chains are eliminated by providing a single pair of spaced apart carriage lift jacks. This however, is only a partial solution to the problem in that only partial free lift is provided and not full free lift of the carriage. Through a reading of the specification and inspection of the drawings of the '893 patent, we can see that the stroke length of the carriage lift jacks would be enough to only raise the carriage equal to that distance and no more. Therefore, the maximum amount of carriage lift is equal to only one half the length of the second pair of inner uprights (movable). Therefore, only partial free lift is provided.
The '893 patent discloses the use of a mast lift jack with a ported rod for delivering pressurized fluid flow to the carriage lift jacks. Leakage fluid passing from the rod end of the carriage lift jacks to the head end of the carriage lift jacks would result in impaired operation of the carriage lift jacks and eventual uneven operation. If leakage past the piston should continue over a prolonged period of time and not be able to escape, the operation of the jack will diminish to the point where the stroke length will be drastically reduced. Thus, the free lift of the carriage will be reduced an equal amount. As the amount of leakage in the carriage lift jacks differ, the carriage, which is guided by rollers on the second pair of uprights, will become cocked which will cause adverse side loading and premature ware of the uprights, carriage and the like in the same manner as discussed above. This cocking will also cause difficulty in alignment of the carriage mounted forks with the load to be lifted, as discussed above. Therefore, it is necessary to eliminate uneven carriage lift jack operation caused by fluid leakage.
In applications where one or more jacks (carriage, upright) are provided for each of the moveable elements (carriage, second pair of uprights, etc.) sequencing of operation of the lift jacks often gets out of order. As a result the second pair of uprights may move before the carriage which causes overhead clearance problems and defeats the primary purpose of the full free lift provision. Attempts to solve this problem by the use of sequence valves and the like have resulted in limited success. However, cost, complexity and reliability of such systems result in infrequent use. Therefore a simple, economical, and reliable solution to the problem is needed.
In summary, it is desirable to provide a lift mast assembly wherein full free lift is obtained without the need for carriage lift chains and the like, and one in which the solution eliminates the problems of unequal jack operation, reduced free lift, improper carriage lift jack and upright lift jack sequencing, and premature wear of the carriage lift jack, the carriage, the lift mast uprights and associated components.